This invention relates generally to a method and an apparatus for measuring corrosion and more particularly to a method and an apparatus for measuring the corrosion beneath thin films of material.
The destructive effects of corrosion are well known and efforts to minimize those effects are ongoing. The naval aircraft-carrier environment is a particularly corrosive one, with sulfur from aircraft-carrier stack gases combining with sea spray to provide a hostile environment which undermines the structural integrity of naval aircraft. Corrosion of aircraft hydraulic pistons, engine components, and bearings is a particularly serious problem. The corrosion protection of such parts relies greatly on the formation and stability of thin lubricant films which remain on the metal surfaces after the lubricant drains, and act as barriers to the hostile environment. The corrosion-inhibiting ability of these thin lubricant films is therefore of considerable importance. For example, naval aircraft turbine engine bearings have exhibited severe corrosion problems due to the harsh aircraft-carrier environment and have relied on lubricant for corrosion protection. Work has been done in the area of developing lubricant oils for these bearings which have the necessary thermal stability for such a high temperature application without sacrificing good corrosion-inhibiting ability. Efforts have also been made to develop additives to make existing oils more corrosion-inhibitive. Means are necessary for testing the effectiveness of these new lubricants as corrosion inhibitors. Such a testing means would be useful in testing lubricants for other applications such as automobiles, as well as in testing other liquid films as corrosion inhibitors.
The traditional method of measuring the corrosion-inhibiting ability of a lubricant consists of placing a lubricant-coated specimen in a standard humidity cabinet and measuring the amount of time required for formation of one or more rust spots. Another commonly used method is to expose lubricant-covered or painted metal panels to tropical or sea environments and then to measure the length of time required for initiation of visible corrosion. These methods lack good reproducibility and are not quantitative. They also rely on visual identification of spots, whereas corrosion is actually present before it becomes visible.
A more recent development in measuring the corrosion-inhibiting characteristics of lubricants involves suspending a lubricant-coated steel test specimen over boiling acidified water and measuring the weight change in the specimen as a function of hours of exposure. Although this method is quantitative, the corrosive test conditions represent an acidic type of corrosion process not reflective of the type of corrosion which actually occurs in typical marine or tropical environments. These methods also lack sufficient sensitivity to detect the slight changes in corrosion rates that occur in response to changes in environmental conditions.
Galvanic cell-type corrosion probes are currently used to determine the corrosivity of a surrounding medium. Such probes have been used on naval aircraft carriers to monitor the corrosivity of the sea environment due to moisture and salt and are used in the absence of any protective films. In operation, a thin film of water from the environment in question, usually in the form of sea spray, contacts the bare probe, acting as an electrolyte to complete the galvanic cell formed by two dissimilar metals in the bare probe separated by insulation. In other words, this film of water acts as a pathway for current flow between the surfaces of the dissimilar metals. Such probes have only been used bare and only to measure the corrosivity of the environment. Their use for measuring the corrosion-inhibiting abilities of coatings has not been contemplated, nor are they structurally suitable for such use.